The wind power plants (WPP) having wind generators, to which the building methods according to the invention refer, comprise a concrete tower and heavy, partly widely projecting large components mounted on the spire. These large components, if any, include a nacelle, an electric generator, a rotor having at least one rotor blade, and, if any, a transmission for the rotor as well as in the case of hybrid towers, additional tubular steel segments which are placed onto the concrete tower and support the wind generator(s).
Wind power plants of different sizes, powers and of different types are increasingly used to generate electric energy from the kinetic energy of the wind. The efficiency of such a wind power plant depends, among other things, on the fact that the wind is present and distributed as long and as uniformly as possible throughout the year.
It is known that the yields which can be produced by wind power plants from the wind supply distributed over the year are the larger the higher the wind power plants can be constructed, as in higher heights the wind on average blows faster and in a more laminar fashion. This applies in particular to inland regions or to hilly or mountainous regions.
In the recent years, the trend is moving towards ever increasing plant units due to economic considerations, with the most widespread type of wind power plants, the type with a multi-blade rotor having a horizontal axis positioned on a tower. This type still has the largest market potential. One problem of this type of wind power plant consists in that the upscale of large components of the wind generator simultaneously necessitates an increasingly higher and stabler tower. In the meantime, 30 to 45% of the total building costs are allotted to these tower structures and their foundations, which makes the economic efficiency of the tower constructions of such large wind power plants (“multi-megawatt plants”) to a decisive factor. Accordingly, this also applies to WPPs having rotors with a vertical axis, the size of which is also constantly increasing.
In principle, WPPs which are increasingly high and increasingly powerful are practical, since even in the conventional building principle of the self-supporting vertical cantilever arm fixed at one end, the materials consumed for the tower structure rise merely to the second power as the performance increases. The energy yield leads to an increase of the yield to the third power due to the performance equation of the wind. In the past, this circumstance has led to a considerable increase in performance of WPPs, which is still incessant.
This trend has also been reflected in the development of increasingly high and strong towers. However, in case of rotors having a horizontal axis, this trend now meets a limit at which the additional yield of the machine considerably restricts the overall economic efficiency due the significantly increasing tower and foundation costs. In particular the operators of wind power plants on sites with a low wind availability, for example on inland sites, depend on very high hub heights to economically generate yields. Thus they depend on very high and simultaneously very economical tower structures.
In recent years, different tower types have been tested and developed to increase the hub height and to shift the rotor to higher atmospheric layers with improved wind availability. Here, in particular the concrete tower, the hybrid tower comprising a concrete shaft and a fitted steel mast on which the wind generator is placed, as well as high lattice mast constructions are to be mentioned.
Each of these tower types has its own advantages and drawbacks, the one or other type of tower being then preferred depending on the case of application.
A drawback in all building methods for powerful wind generators offered on the market is the high increase in material consumption for static and dynamic reasons, which is caused by the building principle as such: the higher the freestanding towers are configured, the more the constructive effect becomes important. The mass consumption increases exponentially with an increasing height like mentioned above. On the one hand this effect results from a consideration of the lever principle of statics, according to which the product of force and lever arm causes a moment in the component which necessitates an appropriate dimensioning. On the other hand it results from the requirements of dynamics, according to which components have to be provided with appropriate reserves in the cross-section as a result of fatigue stress over the period of use. In towers of wind power plants, this effect is increasingly noticeable in particular as from heights of approximately 120 meters.
Furthermore, as from approximately 140 meters, additional inherent difficulties immanent in material and construction occur in all tower building methods. They are also to be mentioned here to illustrate on the basis of which problem the invention has been developed:
Pure tubular steel towers having a solid web cross-section are particularly susceptible to oscillations as from heights of 120 meters due to their softness. Absorbers and damping elements are necessary to reduce the oscillations of the tower. In the case of pure steel constructions, for example, at a hub height of 140 meters, up to two thirds of the required steel amount are furthermore necessary to respond to the dynamic stresses and the problems of fatigue and the oscillation excitation to be avoided. Moreover, due to the mentioned softness, specific rotor speeds cannot be used since they would incite oscillations of the tower. Due to this fact, at specific wind velocities, the rotor has to rotate more slowly than the wind would permit. This results in a loss of performance outside the speed optimum for the given wind profile.
Concrete towers and hybrid towers require, as the height increases, considerable masses as a result of the required tower shaft spread at the lower end of the tower. They require considerable prestressing forces necessary to configure the tower which constitutes a lever arm. Here, cast-in-situ towers and prefabricated towers bonded by horizontal joints or having unstuck horizontal dry joints function with extreme prestress forces. This means that the prestress applied via the tension members is adjusted to provide a sufficient counterforce to the tensile forces occurring in the component as a result of the lever arm. The prestress force is dimensioned in order to maintain the concrete shaft optionally constantly prestressed up to the extent that joints would not open, or such that in case of cast-in-situ towers, a transition from state I to state II of the concrete is permitted only under extreme load. In case of cast-in-situ constructions or stick prefabricated tower-parts the transition from state I to state II leads to additional shifts of natural frequency of the tower.
Though lattice masts first avoid the considerably rising mass increase of the already mentioned building methods as the height increases is implemented by resolving the tower shaft into individual bars. In lattice towers the forces occurring are transmitted by a lattice structure of composed bars between the nacelle and the ground area. Lattice masts however have a particularly wide spread at their lower end due to their building principle. This spread is often felt as unpleasant. In consequent lattice masts have never become widely accepted. Furthermore, they are classified as no ideal tower building method by the maintenance staff of the WPP because there is no weatherproof climbing means to the generator. This is making their work hard during all the years of operation. High lattice mast towers are moreover considered as prone to torsion.
The drawback in all cantilevers fixed at one end are all the natural frequencies of the freestanding tower structure resulting from the susceptibility to oscillation, which are illustrated in Campbell diagrams and which lead to the described power losses during operation of the plant. Specific speed ranges have to be avoided due to their natural frequency, or they require a modified (increased) tower geometry for the desired nominal operation which does not constitute the economical optimum with respect to the tower design. Today, most towers are configured as so-called “hard/soft” constructions in which the permitted speed range of the rotor begins in a Campbell diagram above the intersection between the first natural frequency of the tower structure and the excitation by the rotor 3p having usually 3 blades. The speed range ends before the excitation p is reached, which is for example caused by unbalances of the rotor. A very high material consumption to reach a sufficient rigidity for the tower construction is a “hard/hard” design of the tower construction in which the lower speeds needs to be avoided. Lower speeds, however, are desirable for sites with light winds. Soft/soft constructions as a third design option are rejected by the experts as being insufficiently determined.
At the beginning of the development of commercial wind power industry, a guyed tower tube made of steel has been tested at the approximately 100 meters high tower of the large wind plant Growian and at the smaller experimental plant Monopteros in Wilhelmshaven. The configuration constituted a certain improvement of the static system but it was not further pursued due to multiple technical problems of both plants. At that time, the guying was guided up to below the nacelle, and the rotor blades were provided with an appropriate inclination to the front. This is no longer acceptable with respect to the rotor blade geometry according to today's building principles which are to be as economical as possible. Today, in an extreme load-case, the blade tips run very close to the tower.
Today, guyed tower tubes are merely offered and used in the field of small wind power plants and for plants of middle size. They have a low hub height of less than 100 meters. They usually consist of a metallic tube which is guyed by ropes below the rotor. The purpose of this guying is merely restricted to the removal of the forces acting horizontally on the tower shaft, in particular as a result of the occurring rotor thrust that needs to be transferred into the subsoil. In this case of application, this building method is economical due to the highly reduced application of moment at the base point of the shaft construction and the unnecessary spreading of the shaft tube at the base of the tower.
One characteristic of the guying point at the tower shaft however is the laterally displaceable and flexible torsion spring. This makes the tower shafts made of steel torsionally very weak, in particular from an overall height of 150 meters, as far as they are also configured very slim. The drawback of the torsional weakness becomes important in combination with the now usual blade lengths of more than 60 meters for the mounted wind generators. Unevenly flowing wind at a vertical wind shear (more wind on the left than on the right), in case of turbulences or wind shadow effects in wind farms or of laterally rotating winds with an inclined flow generate a lever force. E.g. the left side of the rotor receives more thrust than the right side. In case of rotor diameters in particular of more than 120 meters the lever force becomes considerable. This lever force is received by the rotating blades is transmitted by the nacelle as torsional force into the tower shaft. These torsional forces, in addition to the lever forces acting on the tower shaft as dynamic stresses, play an increasingly important role the more the component size increases. The invention does not prefer the guying of pure steel tower shafts for the reason that the bending natural frequency and the torsional natural frequency of the tower and the excitation frequency of the large components of the wind generator, are too close to each other. Despite the required and desired slenderness of the tower construction, a low level of oscillation is a core criterion for a modern WPP with respect to its performance, since all components connected to the tower must ensure their full operational safety also in the worst load-case. In particular it must not cause a collision between the blade and the tower even in case of maximum unfavorable deflection of several components. Consequently, in reverse, this also leads to decreasing power-yields by switching-offs or suboptimal operating states. With regard to the invention explained further below, today load limits and excitation frequencies or similar of the rotor-blades and of the tower restrict the maximum possible wind-yield of the WPPs. Solid web steel constructions which tend to be softer in terms of bending and torsion and the material resistance are therefore not suitable for powerful wind generators having slender and long component dimensions.
The same applies to lattice masts. Slender guyed constructions such as transmitter masts or wind measuring masts, cannot support considerable weights and laterally widely projecting top charges.
Further, conventional fixed and guyed constructions of WPPs have no properties, e.g. actuators to alternate the prestressing of the guy members. No property influences the rigidity or the possible deflection of the tower structure to hold the generator in an improved manner with the rotating mass. Here, in particular the aerodynamic damping is to be mentioned. With aerodynamic damping in conventional building methods, the tower always responds in the same manner and does not permit any variably adjustable response behavior to the applied loads, in particular to the loads applied by the rotor thrust at the top flange. Conventional tower structures always respond in the same manner only because of the preset material properties, inertia and the overall configuration of the construction. This behavior results in a premature fatigue or in reverse to a higher material consumption both in the tower structure and in the rotor blades. In the end more material is used to obtain the desired properties with respect to elasticity, rigidity, oscillation behavior and the excitation frequencies of the components to be avoided, i.e. the aeroelastic interaction of rotor blades, turbine and tower structure as a unit.
In case rotors having a vertical axis are used, strong exciting periodic frequencies are disadvantageous due to the rotation of the rotor blades about the longitudinal axis. As a result of the usual rigidity of the rotor blades of this type of plant, the forces introduced into the tower are many times higher than comparable forces in multi-blade devices having a horizontal axis. The tower structure is much more loaded and tends to rolling or wobbling motions similar to precession as a result of the applied forces, which can no longer be economically compensated by a pure increase in material. The problem is further intensified as far as the rotor blades change their angle with respect to the rotation axis, as seen in a top view, at each rotation by means of a pitch system. Pitch systems are in particular used in large rotors having a vertical axis, which makes an adaptation of the tower and the response behavior thereof in particular to the applied periodic and also non-periodic loads of the vertical WPPs all the more necessary.
In conclusion the vertically configured conical cantilever fixed at one end with a maximum load introduction at the upper end reaches an economic limit from heights of 150 meters and more, and the guyed tower types in a steel building method developed so far give no economically relevant answer to the requirements of large multi-megawatt WPPs, in particular of such plants having large blade lengths in regions of light winds, which are very slender and prone to oscillations.
New advantageous tower building methods are now proposed for tower structures preferably as from a hub height of 140 meters against the prejudice of experts. Up to now, it was considered as non-economical, not logical and not in accordance with the trend of the WPP tower construction to propose guyed constructions instead of the freestanding towers for plants of the multi-megawatt class. In particular they were not considered in the context of concrete towers, because these are said to be too heavy. The trend towards increasingly large and high towers has up to now exclusively been organized by an upscale of existing building methods. According to their opinion, the physics of a rope static and the properties of concrete structures, taking the requirements on a highly dynamically loaded WPP tower into account, are said to lead to no economical or practical solution. The additionally oscillating large component of a guy element would make the calculation more difficult, would make the building process unnecessarily more complicated, and would require the maintenance of a further component over the lifetime. Furthermore, the interaction between the frequencies of guy elements and the other components of a WPP are not economically controllable. For these reasons, the existing building methods are maintained.
The invention shows that this is not correct.